Our long-term goals are to understand the mechanism and biological functions of DNA topoisomerase IB (ToplB). The ToplB family includes eukaryotic nuclear and mitochondrial ToplB, poxvirus and mimivirus topoisomerases, and the poxvirus-like topoisomerases of bacteria. ToplB enzymes relax DNA supercoils by breaking and rejoining one strand of the DNA duplex. They act via a transesterification mechanism involving a covalent DNA-(3'-phospho-tyrosyl)-enzyme intermediate. This laboratory uses vaccinia virus as a model system to study ToplB. The vaccinia-encoded ToplB is packaged within the virus particle, where it plays a critical role in replicative fitness by aiding viral mRNA synthesis. A distinctive feature of the poxvirus ToplB is its specificity in forming a covalent intermediate at a target sequence 5'-(C/T)CCTT. All poxvirus topos recognize this site, as does the homologous mimivirus ToplB enzyme. We hypothesize that DNA target recognition triggers the recruitment of catalytic amino acid side chains to form the ToplB active site. An aim of this project is to elucidate at single-atom resolution the structural basis for DNA transesterification and target site specificity and to define the conformational steps for active site assembly and supercoil relaxation. This will be accomplished by an innovative multidisciplinary approach involving DNA chemistry, protein modification with non-natural amino acids, and single-molecule studies, along with "classical" structure-guided mutagenesis and biochemistry. We also aim to dissect genetically which properties of vaccinia ToplB are important in vivo, by gauging the effects of biochemically characterized ToplB mutations on vaccinia virus replication. Relevance: Understanding the catalytic mechanism of ToplB is a high priority because: ToplB is implicated in virtually every DNA transaction in human cells;nuclear ToplB is the target of anticancer drugs that exert their cytotoxicity by perverting the cleavage-religation equilibrium;and ToplB enzymes are distributed widely in bacterial and viral pathogens, where they present untapped targets for mechanism-based anti-infective drug discovery. Exploitation of new molecular targets for treatment of poxvirus infections is a pressing issue, given the concerns that smallpox could be used as a bioterror weapon and the risk of complications of vaccinia infections if a prophylactic vaccination program is resumed.